This invention relates to a method for treating a virgin drainage area within a subterranean tar sand formation that is penetrated by a well. The invention increases the amount of viscous petroleum that may be produced by cyclic steam stimulation. More particularly, the invention relates to a production method wherein a virgin drainage area within such a formation is fractured by injecting liquid carbon dioxide and stimulated by injecting steam while carbon dioxide is still in place.
Conventional production techniques are ineffective in many substerranean formations because the oil in place has too high a viscosity. The tar sand formations in Canada and the western United States are examples of such problem formations.
A tar sand comprises an essentially unconsolidated sand formation that is saturated with a highly viscous petroleum. The petroleum found in such formations typically has an API gravity in the range of from about 5.degree. to about 10.degree. and is highly bituminous in character. At formation temperatures and pressures, such petroleum possesses a viscosity that may range as high as a million centipoise. Consequently, the mobility of such petroleum within a tar sand formation is exceedingly low.
In tar sand formations where strip mining is not economically or ecologically feasible, various thermal techniques can be used to stimulate the formation for petroleum production. Such techniques include hot water drive, in-situ combustions and steam stimulation. Of these techniques, steam stimulation is the most widely used.
There are two commonly used steam stimulation techniques. One technique employs a steam drive between a steam injection well and one or more production wells. When the permeability of the formation is too low for adequate steam transmission, communication between the injection and production well may be established by hydraulic fracturing. Various improvements have been made in this steam drive technique. U.S. Pat. No. 3,908,762 discloses a procedure for stabilizing the communication path between an injection and production well by injecting a mixture of steam and non-condensable gas, such as carbon dioxide. The inventor asserts that carbon dioxide gas passing through the channel in the presence of steam prevents channel plugging by preventing the resolidification of bitumen within such channels. U.S. Pat. No. 4,217,956 discloses a steam drive technique that uses pressurization and draw down cycles wherein carbon dioxide gas is injected at the beginning of the pressurization cycle. The inventor asserts that the presence of carbon dioxide within the formation during pressurization-draw down cycling causes a better emulsification of oil with steam condensate and facilitates better transport of the oil to the producing well.
The other steam stimulation technique used to produce petroleum from tar sand formations is commonly known as the "huff-puff" technique. In the huff-puff technique, steam is injected through a single well into the formation in quantities sufficient to increase the temperature of the petroleum within the drainage area of the formation such that the petroleum becomes mobile. When the drainage area is of insufficient original permeability to receive adequate quantities of steam, the transmissibility of the drainage area may be increased by conventional hydraulic fracturing techniques. After the requisite quantity of steam has been placed within the formation, the well is shut-in and the formation allowed to "soak" for a period of time. Thereafter, the well is opened to produce the effluent liquids within the steam treated drainage area. U.S. Pat. No. 4,217,956 notes that production by huff-puff stimulation may be increased by injection of carbon dioxide gas at the beginning of a steaming cycle.
U.S. Pat. No. 3,909,762 notes that both steam stimulation techniques can be used on the same formation. For instance, when a communicating well fracture cannot be established between an injection and production well, each well may be treated by a huff-puff procedure involving injection of steam with a non-condensable gas such as carbon dioxide, until such a communication path is established. Thereafter, the steam drive technique is used to produce the formation.
With huff-puff stimulation procedures, peak petroleum production is obtained during the early production cycles. The production level obtained on the first cycle generally dictates whether subsequent cycles will be economical. Thus, it is important to condition the virgin drainage area of a tar sand formation to prepare it for the maximum possible petroleum production during the first cycle of huff-puff steam stimulated production.